1. Field of the Invention
The present invention relates to an image reading apparatus such as an image scanner, and an image reading lens to be mounted on the image reading apparatus.
2. Description of Related Art
Image reading apparatuses have been heretofore used. In each of the image reading apparatuses, an image of a color document is formed on a solid-state image sensing device such as a CCD (Charge-Coupled Device) through an optics system so that information of the image is read. In recent years, such image reading apparatuses, particularly image scanners and so on have been requested not only to have higher resolution but also to have a smaller size and lower price.
In this background, an image reading lens having a compact configuration of four lenses by way of example has been developed as an imaging optics to be mounted in an image reading apparatus (for example, see JP-A-11-190820 and JP-A-2002-296499). Each image reading lens disclosed in JP-A-11-190820 and JP-A-2002-296499 is referred to as a telephoto type having a front group including a positive lens and a rear group including a negative lens.
The image reading lenses disclosed in JP-A-11-190820 and JP-A-2002-296499 exhibit excellent optical performance when the angle of view is not larger than about 30°. When the angle of view is larger than 30°, the field curvature increases. It is therefore necessary to secure a distance between the document surface and the image reading lens so that the whole of the document is put within the range of the angle of view not larger than 30°. For this reason, in fact, miniaturization of the total configuration is limited by the dimensions of the document. It is therefore desired to provide an image reading lens which is constituted by a small number of lenses so as to be miniaturized, but can read a larger document.